Many types of heavy equipment have a boom which can be extended or retracted. For example, the LIM-MIT.TM. stroke tree delimbing machines manufactured by Risley Equipment Ltd. of Grande Prairie, Alberta, Canada have a pair of delimbing knives mounted at the end of a boom. The operator of the machine can pick up a felled tree with the delimbing knives, and hold one end of the felled tree between feed rolls on the machine. The operator can then extend or retract the boom to sweep the delimbing knives along the felled tree, thereby removing branches from the felled tree. Maintaining high levels of production with a delimber of this type requires that the boom be capable of being extended or retracted very quickly. Typical boom speeds are in the range of 10 to 20 feet per second.
In smaller delimbers having short booms it can be practical to operate the boom with a telescoping hydraulic piston. This becomes impractical as the stroke of the boom gets longer. Larger delimbers may typically require the boom to have a stroke of approximately 30 feet, or more.
In modem larger delimbers the boom is typically driven by means of a drum mounted to the machine above the boom. A length of cable extends from each end of the boom to the drum. The inner end of each length of cable wraps around, and is attached to the drum. The outer ends of the lengths of cable are attached to the front and rear ends of the boom respectively. The drum is grooved to guide the cable onto the drum. The boom can be extended or retracted by rotating the drum in one direction or the other.
A problem with cable driven booms is that the cables are expensive and can wear out very quickly. The cables are subjected to extreme tension which varies very rapidly when the boom is being accelerated. The cables can begin to unwind each time tension is applied. This accelerates wear on the cables. A further problem with a cable drive is that the cables are not always collinear with the boom because the lateral positions of the points at which the cables wrap onto the drum vary as the cables wind onto and off of the drum. When the cables are not always perpendicular to the axis of the drum they wear, and the drum wears, faster than is desirable. The drum is expensive because its surface must be grooved to receive the cable. A cable drive is also noisy and transmits shock to the machine itself when the boom is suddenly stopped.